CodeSOD: Return of the Mask

Sometimes, you learn something new, and you suddenly start seeing it show up anywhere. The Baader-Meinhof Phenomenon is the name for that. Sometimes, you see one kind of bad code, and the same kind of bad code starts showing up everywhere. Yesterday we saw a nasty attempt to use bitmasks in a loop.

Today, we have Michele’s contribution, of a strange way of interacting with bitmasks. The culprit behind this code was a previous PLC programmer, even if this code wasn’t running straight on the PLC.

public static bool DecodeBitmask(int data, int bitIndex)
{
        var value = data.ToString();
        var padding = value.PadLeft(8, '0');
        return padding[bitIndex] == '1';
}

Take a close look at the parameters there- data is an int. That’s about what you’d expect here… but then we call data.ToString() which is where things start to break down. We pad that string out to 8 characters, and then check and see if a '1' happens to be in the spot we’re checking.

This, of course, defeats the entire purpose and elegance of bit masks, and worse, doesn’t end up being any more readable. Passing a number like 2 isn’t going to return true for any index.

Why does this work this way?

Well, let’s say you wanted a bitmask in the form 0b00000111. You might say, “well, that’s a 7”. What Michele’s predecssor said was, "that’s text… "00000111". But the point of bitmasks is to use an int to pass data around, so this developer went ahead on and turned "00000111" into an integer by simply parsing it, creating the integer 111. But there’s no possibly way to check if a certain digit is 1 or not, so we have to convert it back into a string to check the bitmask.

Unfortunately, the software is so fragile and unreliable that no one is willing to let the developers make any changes beyond “it’s on fire, put it out”.

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CodeSOD: A Bit Masked

The “for-case” or “loop-switch” anti-pattern creates some hard to maintain code. You know the drill: the first time through the loop, do one step, the next time through the loop, do a different step. It’s known as the “Anti-Duff’s Device”, which is a good contrast: Duff’s Device is a clever way to unroll a loop and turn it into a sequential process, while the “loop-switch” takes a sequential process and turns it into a loop.

Ashlea inherited an MFC application. It was worked on by a number of developers in Germany, some of which used English to name identifiers, some which used German, creating a new language called “Deunglish”. Or “Engleutch”? Whatever you call it, Ashlea has helpfully translated all the identifiers into English for us.

Buried deep in a thousand-line “do everything” method, there’s this block:

if(IS_SOMEFORMATNAME()) //Mantis 24426
{
  if(IS_CONDITION("RELEASE_4"))
  {
    m_BAR.m_TLC_FIELDS.DISTRIBUTIONCHANNEL="";
    CString strKey;
    for (unsigned int i=1; i<16; i++) // Test all combinations
    {
      strKey="#W#X#Y#Z";
      if(i & 1)
        strKey.Replace("#W", m_strActualPromo);  // MANTIS 45587: Search with and without promotion code
      if(i & 2)
        strKey.Replace("#X",m_BAR.m_TLC_FIELDS.OBJECTCODE);
      if(i & 4)
        strKey.Replace("#Y",TOKEN(strFoo,H_BAZCODE));
      if(i & 8)
        strKey.Replace("#Z",TOKEN(strFoo,H_CHAIN));

      strKey.Replace("#W","");
      strKey.Replace("#X","");
      strKey.Replace("#Y","");
      strKey.Replace("#Z","");

      if(m_lDistributionchannel.GetFirst(strKey))
      {
        m_BAR.m_TLC_FIELDS.DISTRIBUTIONCHANNEL="R";
        break;
      }
    }
  }
  else
    m_BAR.m_TLC_FIELDS.DISTRIBUTIONCHANNEL=m_lDistributionchannel.GetFirstLine(m_BAR.m_TLC_FIELDS.OBJECTCODE+m_strActualPromo);
}

Here, we see a rather unique approach to using a for-case- by using bitmasks to combine steps on each iteration of the loop. From what I can tell, they have four things which can combine to make an identifier, but might get combined in many different ways. So they try every possible combination, and if it exists, they can set the DISTRIBUTIONCHANNEL field.

That’s ugly and awful, and certainly a WTF, but honestly, that’s not what leapt out to me. It was this line:

if(IS_CONDITION("RELEASE_4"))

It’s quite clear that, as new versions of the software were released, they needed to control which features were enabled and which weren’t. This is probably related to a database, and thus the database may or may not be upgraded to the same release version as the code. So scattered throughtout the code are checks like this, which enable blocks of code at runtime based on which versions match with these flags.

Debugging that must be a joy.

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CodeSOD: CONDITION_FAILURE

Oliver Smith sends this representative line:

bool long_name_that_maybe_distracted_someone()
{
  return (execute() ? CONDITION_SUCCESS : CONDITION_FAILURE);
}

Now, we’ve established my feelings on the if (condition) { return true; } else { return false; } pattern. This is just an iteration on that theme, using a ternary, right?

That’s certainly what it looks like. But Oliver was tracking down an unusual corner-case bug and things just weren’t working correctly. As it turns out, CONDITION_SUCCESS and CONDITION_FAILURE were both defined in the StatusCodes enum.

Yep- CONDITION_FAILURE is defined as 2. The method returns a bool. Guess what happens when you coerce a non-zero integer into a boolean in C++? It turns into true. This method only ever returns true. Ironically, the calling method would then do its own check against the return value, looking to see if it were CONDITION_SUCCESS or CONDITION_FAILURE.

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